The present invention relates to a semiconductor laser, a semiconductor device and their manufacture methods, and more particularly to a semiconductor laser and a semiconductor device having a semiconductor region of a low dislocation density, and to their manufacture methods.
Active developments on light emitting devices using GaN based materials are being made nowadays. Blue and green high luminance light emitting diodes (LED) have been manufactured to date. Oscillation of a royal purple laser at a room temperature has been realized by many research organizations including the present inventor, and studies of manufacturing the products of this laser are made vigorously. A GaN based laser using a sapphire (Al2O3) substrate was manufactured and continuous wave oscillation (CW oscillation) during 1000 hours was confirmed (refer to S. Nakamura et al., Japanese Journal of Applied Physics, vol. 35, p. L74, 1996).
A manufacture method for a short wavelength semiconductor layer using a sapphire substrate will be described briefly. First, an a sapphire substrate having (0001) plane as its principal surface, a GaN buffer layer is formed at a low temperature. A method of forming this GaN buffer layer will be described with reference to FIGS. 39 to 42.
As shown in FIG. 39, on the principal surface of the (0001) plane of a sapphire substrate, a GaN layer 201 is grown to a thickness of 1 to 2 xcexcm by metal organic vapor phase epitaxy (MOVPE). On the surface of the GaN layer 201, a SiO2 film is deposited to a thickness of 100 to 300 nm by chemical vapor deposition (CVD). This SiO2 film is patterned by using hydrofluoric acid to leave striped SiO2 patterns 202. After the SiO2 film is patterned, the substrate surface is cleaned satisfactorily with water.
As shown in FIG. 40, a GaN layer is grown on the substrate surface by MOVPE. At the initial growth stage, a GaN layer 203 is grown only in an area where the GaN layer 201 is exposed. As the growth of the GaN layer continues, as shown in FIG. 41 a GaN layer 204 starts being deposited also on the SiO2 pattern 202.
As the growth continues further, adjacent GaN layers contact each other and the GaN layer covers the whole substrate surface. A GaN buffer layer 205 having generally a flat surface can be formed eventually, as shown in FIG. 42.
FIG. 43 is a schematic diagram showing the state of dislocations in the GaN buffer layer 205. Because of lattice mismatch between sapphire and GaN, dislocations 206 and 207 extend from the interface between the sapphire substrate 200 and GaN layer 201 into the GaN layer 201. The dislocation 206 in the region where the SiO2 pattern 202 is formed does not extend above the SiO2 pattern 202. In the region where the SiO2 pattern 202 is not formed, the dislocation 207 extends into the GaN buffer layer 205.
A region 208 above the SiO2 pattern 202 was formed by a lateral growth of GaN. Therefore, dislocation does not enter this region 208 above the SiO2 pattern 202, and the dislocation density in this region 208 becomes low.
As shown in FIG. 44, a SiO2 pattern 209 and a GaN buffer layer 210 may be formed by repeating the processes shown in FIGS. 39 to 42. In this case, as viewed along a substrate normal line direction, the SiO2 pattern 209 is disposed approximately superposed upon the region where the SiO2 pattern 202 is not disposed.
Extension of the dislocations 207 in the GaN buffer layer 205 is stopped by the SiO2 pattern 209. It is therefore possible to form the second-layer GaN buffer layer 210 having a low dislocation density on the GaN buffer layer 205. With this manufacture method, although the dislocation density of the GaN buffer layer can be lowered, the number of processes increases so that the manufacture cost rises.
Next, a method of forming a laser structure on a GaN buffer layer will be described. On the GaN layer, a laminated structure is formed including an n-type GaN intermediate layer, an n-type Al0.09Ga0.91N clad layer, an n-type GaN light guide layer (separated confinement hetero structure (SCH) layer), an InGaN multiple quantum well layer, a p-type Al0.18Ga0.82N overflow preventing layer, a p-type GaN light guide layer, a p-type Al0.09Ga0.91N clad layer, and a p-type GaN contact layer. These layers are grown, for example, by MOVPE.
The p-type GaN contact layer and p-type AlGaN clad layer are partially dry-etched to leave a ridge structure. The n-type GaN intermediate layer is partially exposed in an area where the ridge structure is not left. A SiO2 film is formed covering the whole substrate surface. This SiO2 film is patterned to expose a partial upper surface of the ridge structure and a partial surface of the n-type GaN intermediate layer. On the exposed surface of the ridge structure, a p-side electrode is formed having a two-layer structure of Ni/Au. On the exposed surface of the n-type GaN intermediate layer, an n-side electrode is formed having a two-layer structure of Ti/Au. Lastly, a pair of parallel side surfaces constituting resonator side surfaces is formed by dry etching.
The resonator side surfaces are formed by dry etching because it is difficult to cleave a sapphire substrate. Flatness of the resonator side surfaces formed by etching is worse than those formed by cleavage. Therefore, a threshold current of a short wavelength semiconductor laser using a sapphire substrate becomes larger than that of a semiconductor laser whose resonator side surfaces are formed by cleavage. For example, the threshold current density of the semiconductor laser formed by the above method is about 3.6 kA/cm2.
The n-side electrode cannot be formed on the bottom surface of the sapphire substrate because sapphire has no electric conductivity. It is therefore necessary to expose the surface of the n-type GaN intermediate layer and form the n-side electrode on this exposed surface.
In order to solve the problems essentially associated with using a sapphire substrate, it has been proposed to use a SiC substrate (refer to A. Kuramata, K. Domen, R. Soejima, K. Horono, S. Kubota and T. Tanahasi, Japanese journal of Applied Physics Vol. 36 (1997) L1130, and G. E. Bulman et al, Device Research Conference IV-B-8, 1997).
With reference to FIG. 45, a method of manufacturing a semiconductor laser using a SiC substrate will be described.
A hexagonal 6H-SiC substrate 231 is prepared which has a (000.1) Si plane as its principal surface. The SiC substrate 231 is given n-type conductivity. Sequentially grown by MOVPE on the surface of the SiC substrate 231 are an n-type Al0.1Ga0.9N buffer layer 232, an n-type GaN buffer layer 233, an n-type Al0.09Ga0.91N clad layer 234, an n-type GaN light guide layer 235, an InGaN multiple quantum well layer 236, a p-type Al0.18Ga0.82N electron block layer 237, a p-type GaN light guide layer 238, a p-type Al0.09Ga0.91N clad layer 239, and a p-type GaN contact layer 240.
The AlGaN buffer layer 232 is 0.15 xcexcm thick, the GaN buffer layer 233 is 0.1 xcexcm thick, the AlGaN clad layer 234 is 0.5 xcexcm thick, and the GaN light guide layer 235 is 0.1 xcexcm thick. These n-type layers are doped with Si impurities at a concentration of 3xc3x971018 cmxe2x88x923.
The InGaN multiple quantum well layer 236 has the lamination structure of four barrier layers of undoped In0.03Ga0.97N and three well layers of undoped In0.15Ga0.85N alternately stacked. The barrier layer is 5 nm thick and the well layer is 4 nm thick. Five barrier layers each having a thickness of 5 nm and four well layers each having a thickness of 2.6 nm may also be used.
The AlGaN electron block layer 237 is 20 nm thick, the GaN light guide layer 238 is 0.1 xcexcm thick, the AlGaN clad layer 239 is 0.5 xcexcm thick, and the GaN contact layer 240 is 0.2 xcexcm thick. These p-type layers are doped with Mg impurities at a concentration of 5xc3x971019 cmxe2x88x923.
The p-type GaN contact layer 240 and p-type AlGaN clad layer 239 are partially etched to leave a ridge 241 long in one direction. The ridge 241 is 3.5 xcexcm wide. On the bottom surface of the SiC substrate 231, Ni, Ti and Au are sequentially deposited to form an n-side electrode 243. The surfaces of the ridge 241 and p-type AlGaN clad layer 239 are covered with a SiO2 film 242.
An opening is formed through the SiO2 film 242 to expose an upper surface of the ridge 241. On the exposed surface of the ridge 241 and on the SiO2 film 242, Ni, Ti and Au are sequentially deposited to form a p-side electrode 244. The substrate is cleaved to form a GaN based semiconductor laser having the resonator length of 700 xcexcm.
Since a SiC substrate can be cleaved, a high performance optical resonator can be manufactured easily. Since SiC has electric conductivity, the electrode can be disposed on the bottom surface of the substrate. The device structure can therefore be simplified. Since a difference of lattice constants between SiC and GaN is small, it is possible to epitaxially grown a GaN layer having a small lattice defect density. Since a heat dissipation coefficient of SiC is larger than that of sapphire, use of a SiC substrate is effective for improving the thermal dissipation characteristics.
By using the dislocation reduction method described with FIGS. 39 to 42, a GaN buffer layer having a low dislocation density can be formed for a semiconductor layer using a SiC substrate. The SiO2 pattern 202 used by the dislocation reduction method is an insulating member so that current will not flow through the SiO2 pattern 202 and the device resistance increases.
It is an object of the present invention to provide a semiconductor laser and its manufacture method, capable of suppressing an increase of a device resistance and lowering a dislocation density.
It is another object of the present invention to provide a semiconductor device having a semiconductor region of a low dislocation density and its manufacture method.
According to one aspect of the present invention, there is provided a semiconductor laser comprising: a substrate made of SiC; a plurality of AlxGa1xe2x88x92xN patterns (0xe2x89xa6xxe2x89xa61) formed on a surface of the substrate and dispersively distributed in an in-plane of the substrate; an AlyGa1xe2x88x92yN buffer layer (y less than x) covering the surface of the substrate and the AlxGa1xe2x88x92xN patterns; and a laser structure formed on the AlyGa1xe2x88x92yN buffer layer.
According to another aspect of the present invention, there is provided a method of manufacturing a semiconductor laser, comprising the steps of: growing an AlxGa1xe2x88x92xN layer (0xe2x89xa6xxe2x89xa61) on a substrate made of SiC; selectively etching the AlxGa1xe2x88x92xN layer to partially expose the substrate; selectively growing an AlyGa1xe2x88x92yN buffer layer (y less than x) by using a remaining portion of the AlxGa1xe2x88x92xN layer as seed crystals; and forming a laser structure on the AlyGa1xe2x88x92yN buffer layer.
Since the AlGaN buffer layer is grown by using the AlGaN patterns as seed crystals, a dislocation density of a predetermined region in the AlGaN buffer layer can be lowered. The characteristics of a laser structure can be improved by forming the laser structure above the region having a low dislocation density. Since the AlGaN pattern has electric conductivity, the device resistance can be suppressed from being increased.
According to another aspect of the present invention, there is provided a method of manufacturing a semiconductor laser, comprising the steps of: depositing a silicon oxide film on a substrate made of SiC, selectively etching the silicon oxide film to partially expose a surface of the substrate; selectively growing an AlxGa1xe2x88x92xN layer (0 xe2x89xa6xxe2x89xa61) on the exposed surface of the substrate; removing a remaining portion of the silicon oxide film; selectively growing an AlyGa1xe2x88x92yN buffer layer (y less than x) by using the selectively grown AlxGa1xe2x88x92xN layer as seeds; and forming a laser structure on the AlyGa1xe2x88x92yN buffer layer.
Since the AlyGa1xe2x88x92yN buffer layer is grown by using the AlxGa1xe2x88x92xN layer as seed crystals, a dislocation density of a predetermined region in the AlxGa1xe2x88x92xN buffer layer can be lowered. The silicon oxide film can be removed by wet etching. If the silicon oxide film is etched by wet etching, the surface of the underlying exposed SiC substrate is damaged less. Crystallinity of the AlyGa1xe2x88x92yN buffer layer can be improved further.
According to another aspect of the present invention, there is provided a nitride based group III-V compound semiconductor device, comprising: a semiconductor substrate; a lamination pattern formed on a partial surface of the semiconductor substrate, the lamination pattern including a lower-level layer made of nitride based group III-V compound semiconductor and a higher-level layer made of different material from the nitride based group III-V compound semiconductor; and a grown layer covering the lamination pattern, the grown layer being made of nitride based compound semiconductor easier to be grown on side walls of the lower-level layer than on a surface of the higher-level layer.
According to another aspect of the present invention, there is provided a method of manufacturing a semiconductor device, comprising the steps of: forming a lamination pattern on a partial surface of a semiconductor substrate, the lamination pattern including a lower-level layer made of nitride based group III-V compound semiconductor and a higher-level layer made of different material from the nitride based group III-V compound semiconductor; and selectively growing a growth layer from exposed side walls of the lower-level layer of the lamination pattern by using the lower-level layer as seed crystals, the growth layer being made of nitride based group III-V compound semiconductor.
The grown layer grows by using the lower-level layer of the lamination pattern as seed crystals. Namely, the grown layer is formed by the lateral growth. Therefore, the dislocation density of the grown layer lowers.
According to another aspect of the present invention, there is provided a semiconductor laser comprising: a semiconductor substrate; a buffer layer disposed on a partial surface of the semiconductor laser, the buffer layer being made of group III-V compound semiconductor and including an eaves portion; and a laser structure formed on the buffer layer, an oscillation region of the laser structure being disposed so as to be superposed upon the eaves portion, as viewed along a direction normal to a surface of the semiconductor substrate.
According to another aspect of the present invention, there is provided a method of manufacturing a semiconductor device, comprising the steps of: forming a mask film with openings on a surface of a semiconductor substrate; selectively forming semiconductor buffer regions on surfaces of the semiconductor substrate exposed in the openings, and laterally growing the buffer regions also on surfaces of the mask film near the openings; removing the mask film; and growing a semiconductor layer on surfaces of the buffer regions and the semiconductor substrate.
The buffer region formed on the mask by the lateral growth has a low dislocation density. The semiconductor layer formed on the region having a low dislocation density has also a low dislocation density. As the mask film is removed, the eaves portion is left above the removed region. As compared to covering the whole substrate surface with the buffer region, cracks are hard to be generated in the buffer region.
According to another aspect of the present invention, there is provided a semiconductor laser comprising: a semiconductor substrate; an AlGaN pattern formed on a partial surface of the semiconductor substrate, the AlGaN pattern being made of AlxGa1xe2x88x92xN (0xe2x89xa6xxe2x89xa61), a buffer layer made of AlyGa1xe2x88x92yN (y less than x) and covering a surface of the AlGaN pattern and surfaces of the semiconductor substrate on both sides of the AlGaN pattern; a semiconductor layer covering a surface of the buffer layer and surfaces of the semiconductor substrate on both sides of the buffer layer; and a laser structure formed on the buffer layer, an oscillation region of the laser structure being disposed so as not to be superposed upon the AlGaN pattern as viewed along a direction normal to the surface of the semiconductor substrate.
According to another aspect of the present invention, there is provided a method of manufacturing a semiconductor laser, comprising the steps of: forming an AlGaN pattern on a partial surface of a semiconductor substrate, the AlGaN pattern being made of AlxGa1xe2x88x92xN (0xe2x89xa6xxe2x89xa61); selectively growing a buffer layer made of AlyGa1xe2x88x92yN (y less than x) by using the AlGaN pattern as seed crystals; and stopping the selective growing step before the selectively grown buffer layer covers a whole surface of the semiconductor substrate.
Since the AlGaN buffer layer is grown by using the AlGaN layer as seed crystals, a dislocation density of a predetermined region in the AlGaN buffer layer can be lowered. As compared to covering the whole substrate surface with the AlGaN buffer layer, cracks are hard to be generated in the AlGaN buffer layer.
According to another aspect of the present invention, there is provided a nitride based group III-V compound semiconductor device, comprising: a semiconductor substrate; a lamination pattern formed on a partial surface of the semiconductor substrate, the lamination pattern including a lower-level layer made of nitride based group III-V compound semiconductor and a higher-level layer made of different material from the nitride based group III-V compound semiconductor; and a buffer region made of nitride based compound semiconductor and covering a surface of the lamination pattern and surfaces of the semiconductor substrate on both sides of the lamination pattern, the nitride based compound semiconductor being easier to grow on side walls of the lower-level layer than on a surface of the higher-level layer.
According to another aspect of the present invention, there is provided a method of manufacturing a semiconductor device comprising the steps of: forming a lamination pattern on a partial surface of a semiconductor substrate, the lamination pattern including a lower-level layer made of nitride based group III-V compound semiconductor and a higher-level layer made of different material from the nitride based group III-V compound semiconductor; selectively growing a buffer region made of nitride based group III-V compound semiconductor by using exposed side walls of the lower-level layer of the lamination pattern as seed crystals, and laterally growing the buffer region also on a surface of the higher-level layer of the lamination pattern to cover the surface of the higher-level layer and the partial surface of the semiconductor substrate; and stopping the step of selectively growing the buffer region before the buffer region covers a whole surface of the semiconductor substrate.
Since the buffer region is laterally grown from the side walls of the lower-level layer of the lamination pattern, the buffer region having a low dislocation density can be formed. As compared to covering the whole surface of the semiconductor substrate with the buffer region, cracks are hard to be generated in the buffer region.